Operation of lean burn engines, e.g., diesel engines and lean burn gasoline engines, provide the user with excellent fuel economy, and have very low emissions of gas phase hydrocarbons and carbon monoxide due to their operation at high air/fuel ratios under fuel lean conditions. Diesel engines, in particular, also offer significant advantages over gasoline engines in terms of their fuel economy, durability, and their ability to generate high torque at low speed.
From the standpoint of emissions, however, diesel engines present problems more severe than their spark-ignition counterparts. Emission problems relate to particulate matter (PM), nitrogen oxides (NOx), unburned hydrocarbons (HC) and carbon monoxide (CO). NOx is a term used to describe various chemical species of nitrogen oxides, including nitrogen monoxide (NO) and nitrogen dioxide (NO2), among others.
Oxidation catalysts comprising a precious metal dispersed on a refractory metal oxide support are known for use in treating the exhaust of diesel engines in order to convert both hydrocarbon and carbon monoxide gaseous pollutants by catalyzing the oxidation of these pollutants to carbon dioxide and water. Such catalysts have been generally contained in units called diesel oxidation catalysts (DOC), or more simply catalytic converters, which are placed in the exhaust flow path from a Diesel-powered engine to treat the exhaust before it vents to the atmosphere. Typically, the diesel oxidation catalysts are formed on ceramic or metallic substrate carriers (such as the flow-through monolith carrier) upon which one or more catalyst coating compositions are deposited. In addition to the conversions of gaseous HC, CO and the SOF fraction of particulate matter, oxidation catalysts that contain platinum group metals (which are typically dispersed on a refractory oxide support) promote the oxidation of nitric oxide (NO) to NO2.
Catalysts used to treat the exhaust of internal combustion engines are less effective during periods of relatively low temperature operation, such as the initial cold-start period of engine operation, because the engine exhaust is not at a temperature sufficiently high for efficient catalytic conversion of noxious components in the exhaust. To this end, an adsorbent material, which may be a zeolite, may be provided as part of a catalytic treatment system in order to adsorb gaseous pollutants, usually hydrocarbons, and retain them during the initial cold-start period. As the exhaust gas temperature increases, the adsorbed hydrocarbons are driven from the adsorbent and subjected to catalytic treatment at the higher temperature.
One effective method to reduce NOx from the exhaust of lean-burn engines, such as gasoline direct injection and partial lean-burn engines, as well as from diesel engines, requires trapping and storing of NOx under lean burn engine operating conditions and reducing the trapped NOx under stoichiometric or rich engine operating conditions or lean engine operating with external fuel injected in the exhaust to induce rich conditions. The lean operating cycle is typically between 1 minute and 20 minutes and the rich operating cycle is typically short (1 to 10 seconds) to preserve as much fuel as possible. To enhance NOx conversion efficiency, the short and frequent regeneration is favored over long but less frequent regeneration. Thus, a lean NOx trap catalyst generally must provide a NOx trapping function and a three-way conversion function.
Some lean NOx trap (LNT) systems contain alkaline earth elements. For example, NOx sorbent components include alkaline earth metal oxides, such as oxides of Mg, Ca, Sr and Ba. Other lean LNT systems can contain rare earth metal oxides such as oxides of Ce, La, Pr and Nd. The NOx sorbents can be used in combination with precious metal catalysts such as platinum dispersed on an alumina support in the purification of exhaust gas from an internal combustion engine.
A conventional LNT typically contains basic sorbent components (e.g., BaO/BaCO3 and/or CeO2) for NOx storage and platinum group metals (PGM, i.e., Pt, Pd and Rh) for catalytic NOx oxidation and reduction. The LNT catalyst operates under cyclic lean (trapping mode) and rich (regeneration mode) exhaust conditions during which the engine out NO is converted to N2 as shown in equations 1-6:Lean condition: 2NO+O2→2NO2  (1)(Trapping mode) 4NO2+2MCO3+O2→2M(NO3)2+2CO2  (2)Rich condition: M(NO3)2+2CO→MCO3+NO2+NO+CO2  (3)(Regeneration mode) NO2+CO→NO+CO2  (4)2NO+2CO→N2+2CO2  (5)2NO+2H2→N2+2H2O  (6)
In preparation for the emerging Euro 6 automotive exhaust emission catalyst market to meet increasingly stringent NOx emissions, diesel oxidation catalysts (DOC) for diesel passenger cars may be replaced with a close-coupled lean NOx trap with diesel oxidation functionality (LNTDOC) for engine displacements ranging from 1.2 to 2.5 L. In addition to managing NOx emissions from the vehicle, this change will require the LNTDOC to effectively oxidize engine-out hydrocarbon (HC) and carbon monoxide (CO) emissions. Specifically, this change requires that the LNT fulfill the de-NOx function of converting NOx to N2 while also taking on the dual role of a DOC to oxidize engine-out hydrocarbons (HC) and carbon monoxide (CO) (Equations 7 and 8) and to generate an exotherm for the regeneration of a catalyzed soot filter (CSF).HC and CO oxidation: CxHy+O2→CO2+H2O  (7)2CO+O2→2CO2  (8)
Current LNT technology is not efficient to handle the HC slip during vehicle cold start. The present invention provides an LNTDOC design in order to meet increasingly stringent emissions regulations.